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interval demand could represent the value that will be less than the actual peak
demand of the load, thus enabling the electricity consumer to manipulate the load for
limited periods within the demand interval.
4.3.3 Sliding Window Demand
For sliding window demand, calculations are made upon techniques described above
for sliding averaging. The billing demand interval (demand window) is defined as a
number of short subsequent demand intervals. The PM295 allocates demand storage
registers sufficient for the required number of demand intervals. At the end of the
present demand interval, the information on the oldest demand interval in the
demand window is discarded, and the sliding window demand is calculated as
average on the newest set of demand intervals.
Note that sliding window demand is updated as block demand calculated at the end
of each demand interval. The billing demand period is defined for the sliding
DEMAND PERIOD ×
window demand by two programmable parameters as
NUMBER OF DEMAND PERIODS
.
4.3.4 Thermal Demand
The thermal demand technique is applied to all measured total powers. This method
is similar to the way electro-mechanical thermal demand meters operate. The three-
phase thermal maximum demand measuring element comprises a thermally lagged
power demand pointer with an exponential response to a step change in
instantaneous power load. The PM295 uses numerical techniques to approximate
the thermal response characteristics of the thermal measuring element. A changing
load is approximated by a series of step changes in load by calculating instantaneous
power load at regular 1 second intervals. The thermal demand algorithm used allows
to calculate the thermal response with high accuracy , even for fast changing loads.
For thermal demand calculations, the billing demand period is defined as for the
DEMAND PERIOD × NUMBER OF DEMAND PERIODS
sliding window demand:
.
The additional parameter needed to be defined by the user is a
THERMAL TIME
of the simulated thermal element. A thermal time constant depends on
CONSTANT
billing demand interval and on the electricity tariff rules applied by the power utility
company.
In countries where thermal demand metering is still used, thermal demand meters
may have a thermal time constant such that, for a step change in instantaneous load
from 0 to 100%, the thermal demand pointer will reach 99%, or 63% of its final
Operation Techniques
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